Receiver for quadraphase modulation signals

ABSTRACT

In a receiver of quadraphase modulation signals, a receiver of quadraphase modulation signals which is provided with two kinds of equalizers being different in each sine and cosine phases, together with an equalizer extracting a clock signal.

BACKGROUND OF THE INVENTION

1. Field of The Invention

The subject invention relates to a recorder or a signal transmissionapparatus, and especially to a system of transmission and receptionwhich is excellent in a signal-to-noise ratio.

2. Description of The Related Art

A system called a quadraphase modulation system has been proposed fortransmission of digital signals. A signal subjected to quadraphasemodulation has few low-frequency components, and therefore, it issuitable for the signal transmission in a system, such as a magneticrecording system and a communication system using metal wires, whichdoes not allow low-frequency signals to pass through. This system willbe summarized by using a functional block diagram of FIG. 2 and awaveform diagram of FIG. 3. It is assumed that data ..., a_(2n),a_(2n+1), ... are given from a signal source to an input end of aquadraphase modulation transmitter. These data are converted intoparallel data of an even-number series and an odd-number series by aserial-parallel converter and modulated simultaneously by a sinemodulator and a cosine modulator. When the data after modulation aredenoted by ..., b_(4n), b_(4n+1), b_(4n+2), b_(4n+3), ..., the followingrelationships are established:

    b.sub.4n =a.sub.2n

    b.sub.4n+1 =a.sub.2n+1

    b.sub.4n+2 =-a.sub.2n

    b.sub.4n+3 =-a.sub.2n+1                                    ( 1)

As is seen from these relationships, the series of even orders ofb_(4n), b_(4n+2) are made to correspond to the series of an even orderof a_(2n), while the series of odd orders of b_(4n+1), b_(4n+3) are madeto correspond to the series of an odd order of a_(2n+1) independently ofthe above according to this quadraphase modulation system. When theseries of the even and odd numbers of a_(n) are modulated independentlyas shown in FIGS. 3b and 3c and then added up, accordingly, a waveformafter being subjected to quadraphase modulation as shown in FIG. 3d isobtained. In this case, 0, +1, 0 and -1 covering a period of 2T togetheris a fundamental waveform as shown in FIG. 4a. According to thismodulation system, in other words, every two bits of input data are putin a block and coded, and therefore, one period is composed of two bits.The series of FIGS. 3b and 3c have a phase difference of just 90 degreesfrom each other in relation to this period. Therefore, the former seriesis called sine modulation and the latter cosine modulation.

In a receiver of quadraphase modulation, first the deterioration in afrequency characteristic caused in the course of transmission iscompensated by an equalizer of FIG. 2. Then, the data are binary-codedand the waveform in FIG. 3d is restored. Next, this binary-codedwaveform is discriminated and regenerated at points of ..., S_(4n),S.sub.(4n+1), S.sub.(4n+2), ... and ..., S.sub.(4n+1), S.sub.(4n+2)+1,S.sub.(4n+1)+2, ... shown in FIG. 3e, and thereby data shown in FIG. 3fare obtained. These data are further passed through a parallel-serialconverter, and thereby the original data shown in FIG. 3g areregenerated. The above is the gist of the quadraphase modulation system,and further details thereof are described in IEEE Trans. on Magnetics,Vol. MAG-15, No. 6, 1465-1467, by J. A. Bixby, etc.

The fundamental waveform for cosine modulation according to thequadraphase modulation system is 0, +1, 0 and -1 covering together theperiod of 2T shown in FIG. 4a. This fundamental waveform needs to besubjected to waveform equalization so as not to cause an intersymbolinterference to a code series which is sine-modulated and shifted by T/2therefrom. According to the prior art, the equalization is conducted sothat an impulse response of the above-mentioned fundamental waveform hassuch a waveform as shown in FIG. 4b. When this impulse response isdenoted by Ir(t), in other words, it is given as:

    Ir(O)=+1

    Ir(-T)=-1

    Ir(-nT/2)=0                                                (2)

where n≠0 and n≠1.

An equalized waveform of sine modulation in relation to a fundamentalwave is obtained likewise by shifting an equalized waveform of cosinemodulation by T/2 as indicated by a dotted line. By the equalizationstated above, the original pulse series can be discriminated andregenerated without any intersymbol interference at any points of ...,S_(4n), S.sub.(4n+1), S.sub.(4n+2), S.sub.(4n+3), ... as shown in FIG.4c. For this purpose, accordingly, it is only required to selectsampling points in series of (S_(4n), S.sub.(4n+1)), (S.sub.(4n+1),S.sub.(4n+1)+1) and others or sampling points in series of(S.sub.(4n+2), S.sub.(4n+3)), (S.sub.(4n+1)+2, S.sub.(4n+1)+3) by theprocedures of FIG. 3e, 3f and 3g, for instance, and to discriminate andregenerate the data of each phase.

A frequency band necessary for realizing the waveform of FIG. 3b isdetermined by a pulse width of a fundamental waveform. Now, since thevalue of the width of the fundamental waveform is T/2 as shown in FIG.4a, a Nyquist rate is 1/T. Since the pulse width of the original signalis T, on the other hand, the Nyquist rate in this case is 1/(2T). Inother words, the quadraphase modulation system necessitates atransmission band twice as wide as NRZ. Noise increases as a requiredband widens, and this causes a disadvantage that the number of biterrors also increases.

SUMMARY OF THE INVENTION

The present invention furnishes an optimum method of equalization of afundamental waveform of the quadraphase modulation system and therebysolves the problem of the increase in a frequency band, which is ashortcoming of this modulation system. As is seen from the equation (1),the fundamental waveform after sine modulation has the relationship ofb_(4n) =-b_(4n+2) =a_(2n). When either b_(4n) or b_(4n+2) is known,therefore, the other is determined uniquely therefrom. The same can besaid with regard to cosine modulation as well. Paying attention to thisrespect, the present invention limits the points of discrimination ofdata to one point for the data series of sine modulation or cosinemodulation during the period 2T (said points are two in the prior artsystem), and, instead, gives an impulse response which requires asmaller band than in the prior art system.

BRIEF DESCRIPTION OF THE DRAWINGS

With the above and additional objects and advantages in mind as willhereinafter appear, the invention will be described with reference tothe accompanying drawings, in which:

FIG. 1 is a diagram showing a construction of a receiver for quadraphasemodulation which is one embodiment of the present invention;

FIG. 2 is a diagram showing a conventional example of atransmitter-receiver of a quadraphase modulation system;

FIGS. 3a-3g are waveform diagrams at each part of the quadraphasemodulation system;

FIGS. 4a-4c are waveform diagrams of the quadraphase modulation system;

FIGS. 5a-5d are waveform diagrams showing examples of equalizedwaveforms of the quadraphase modulation system according to the presentinvention;

FIGS. 6a-6b is a diagram showing another example of equalized waveformsof the quadraphase modulation system according to the present invention;and

FIG. 7 is a diagram showing another example of construction of areceiver for quadraphase modulation according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 5a-5d show examples of the impulse response meeting theabove-stated conditions in regard to the fundamental waveform of 0, -1,0 and +1 of cosine modulation. When this impulse response is given by Iw(t): ##EQU1## Values p and q give the amplitudes of the impulse responseat sampling points, respectively. In the case when q>p herein, theimpulse response for sine modulation is obtained by inverting the timebase of the fundamental waveform of cosine modulation as is seen fromFIGS. 5a-5d. In the case when p=q, the fundamental waveform of thecosine modulation needs only to be symmetrical with respect to pointsaround T=0 as shown in FIG. 6, while the fundamental waveform of thesine modulation has a shape obtained by putting forward said waveform inphase by T/2.

When the equalization is so executed as to have such an impulse responseas described above, the point of the time 0, i.e. the sampling pointS_(4n+2), is not affected by the fundamental wave of the cosinemodulation indicated by a solid line as is apparent from FIGS. 5 and 6,and therefore, the data series sine-modulated at this point can bediscriminated and regenerated. The data series cosine-modulated at thetime -T/2, i.e. the sampling point S_(4n+1), is discriminated andregenerated, likewise, without being affected by the sine-modulated dataseries indicated by a dotted line. As the result, the data of FIG. 5dare discriminated and regenerated. What should be noted herein is thatdata of b_(4n+1) =a_(2n+1) are obtained at the sampling point S_(4n+1)and data of b_(4n+2) =-a_(2n) at the sampling point S_(4n+2). That is,the sequence of the regenerated data reverses. Consideration needs to begiven to this respect with regard to a regenerating circuit.

While an interval between zero cross points of the prior-art impulseresponse given by the equation (2) is T, an interval between zero crosspoints of the impulse response of the equation (3) is 1.5 T, and thus itis widened by about 1.5 times or above, at least. Herein, the intervalbetween zero cross points means a time interval between a zero point atthe time 0 of the impulse response and a zero point nearest to saidpoint. As described above, the system of equalization according to thepresent invention can reduce a band necessary for transmission of thequadraphase modulation waveform to about 2/3 of that according to theprior-art system.

The present invention will be described in detail with reference toFIG. 1. Transmitted data are equalized first by an equalizer 1 accordingto the prior-art system shown in FIG. 4, and the data thus equalized aredivided into three and supplied to a transversal filter 2, a transversalfilter 3 and a quadraphase clock signal generator 4. In the transversalfilters 2 and 3, it is only required to convert the waveforms shown inFIG. 4b into those shown in FIG. 5b. This can be realized basically bydeteriorating the high-frequency characteristic of the waveforms of FIG.4b and further by giving phase distortion thereto. In other words, thecoefficients of the transversal filters 2 and 3 may be given as theimpulse response of the frequency characteristic obtained as the resultof dividing the frequency characteristic of the impulse responses of thefundamental waveforms of FIG. 4b by the frequency characteristic of theimpulse response of the fundamental waveform of FIG. 5b. This method iswell known generally. What should be noted herein is that thecoefficients of the transversal filters 2 and 3 are different naturallyfrom each other because of a difference in the impulse response betweenthe fundamental wave of cosine modulation and that of sine modulation asshown in FIG. 5b and therefore, two kinds of filters need to be employedseparately. Next, outputs of these filters are binary-coded atprescribed sampling points by comparators 5 and 6, respectively, andsupplied to a parallel-serial converter 7. In the output of thisconverter, data are given in the sequence of ..., a_(2n+1), -a_(2n) inevery two bits as shown previously, and so the time base is reversed.This time base is inverted by a time base converter 8. As the result,the data are regenerated in the right sequence of ..., -a_(2n),a_(2n+1), .... However, the polarity of the signal is inverted, andtherefore, it is inverted again two bits apart by a signal polarityinverter 9. The original data series is restored correctly by the aboveprocesses.

Besides, the coefficients of the transversal filters 2 and 3 turnidentical in the symmetrical impulse responses shown in FIG. 6.Therefore, a circuit having necessary characteristics can be constructedonly by using one filter 10 as shown in FIG. 7.

According to the present invention, as described above, the quadraphasemodulation waveform can be demodulated in a transmission band of about2/3 of a conventional one, and so data demodulation can be implementedwith an excellent SN ratio. As the result, data communicationaccompanied by small transmission error and having high reliability canbe attained.

Numerous alterations and modifications of the structure herein disclosedwill present themselves to those skilled in the art. However, it is tobe understood that the above described embodiment is for purposes ofillustration only and not to be construed as a limitation of theinvention. All such modifications which do not depart from the spirit ofthe invention are intended to be included within the scope of theappended claims.

We claim:
 1. An information transmission system for transferringquadraphase modulated data signals from a source to a destination,comprising a quadraphase modulation transmitter, a channel, and aquadraphase modulation receiver, the transmitter being provided withquadraphase modulation means for converting even and odd data symbolsinto a stream of even and odd quadraphase channel symbols which aretransmissible from the transmitter to the receiver via the channel, thereceiver being provided with equalizing means for equalizing the channelsymbols and discrimination means for taking decisions with respect tothe equalized channel symbols, characterized in that the equalizingmeans are arranged for producing output pulses having a bandwidthsmaller than the data symbol rate and in that symbol decisions taken bythe discrimination means are reversed in time with respect to the orderin which the data symbols were originally generated.
 2. An informationtransmission system for transferring quadraphase modulated data signalsfrom a source to a destination, comprising a quadraphase modulationtransmitter, a channel, and a quadraphase modulation receiver, thetransmitter being provided with quadraphase modulation means forconverting even and odd data symbols into a stream of even and oddquadraphase channel symbols which are transmissible for the transmitterto the receiver via the channel, the receiver being provided withequalizing means for equalizing the channel symbols, and discriminationmean for taking decisions with respect to the channel symbols,characterized in that the equalizing means comprises two equalizers in aparallel arrangement, said two equalizers being coupled to two separatedetectors forming the discrimination means, one of said two separatedetectors taking symbol decisions with respect to the even channelsymbols, and the other of said two separate detectors taking symboldecisions with respect to the odd channel symbols.
 3. An informationtransmission system according to claim 1, wherein the receiver comprisesa time-base converter for reversing in time the order of the even andodd symbol decisions.
 4. An information transmission system according toclaim 3, wherein the receiver comprises a signal polarity inverter forinverting the polarity of either the even or the odd symbol decisions.5. An information transmission system according to claim 2, wherein thereceiver comprises a time-base converter for reversing in time the orderof the even and odd symbol decisions.
 6. An information transmissionsystem according to claim 5, wherein the receiver comprises a signalpolarity inverter for inverting the polarity of either the even or theodd symbol decisions.
 7. A receiver for receiving quadraphase modulateddata signals, wherein said quadraphase modulated data signals compriseeven and odd data symbols converted into a stream of even and oddquadraphase channel symbols, said receiver being provided withequalizing means for equalizing the channel symbols and discriminationmeans for taking decisions with respect to the equalized channelsymbols, characterized in that the equalizing means are arranged forproducing output pulses having a bandwidth smaller than the data symbolrate and in that symbol decisions taken by the discrimination means arereversed in time with respect to the order in which the data symbolswere originally generated.
 8. A receiver for receiving quadraphasemodulated data signals, wherein said quadraphase modulated data signalscomprise even and odd data symbols converted into a stream of even andodd quadraphase channel symbols, said receiver being provided withequalizing means for equalizing the channel symbols and discriminationmeans for taking decisions with respect to the equalized channelsymbols, characterized in that the equalizing means comprises twoequalizers in a parallel arrangement, said two equalizers being coupledto two separate detectors forming the discrimination means, one of saidtwo separate detectors taking symbol decisions with respect to the evenchannel symbols, and the other of said two separate detectors takingsymbol decisions with respect to the odd channel symbols.
 9. A receiveraccording to claim 7, wherein the receiver further comprises a time-baseconverter for reversing in time the order of the even and odd symboldecisions.
 10. A receiver according to claim 9, wherein the receiverfurther comprises a signal polarity inverter for inverting the polarityof either the even or the odd symbol decisions.
 11. A receiver accordingto claim 8, wherein the receiver further comprises a time-base converterfor reversing in time the order of the even and odd symbol decisions.12. A receiver according to claim 11, wherein the receiver furthercomprises a signal polarity inverter for inverting the polarity ofeither the even or the odd symbol decisions.
 13. An equalizerarrangement for use in a receiver of quadraphase modulation signals,wherein said quadraphase modulated data signals comprise even and odddata symbols converted into a stream of even and odd quadraphase channelsymbols, said equalizer arrangement equalizing the channel symbols,characterized in that the equalizer arrangement is arranged forproducing output pulses having a bandwidth smaller than the data symbolrate.